This invention relates to a process for the surface treatment of titanium and titanium alloys for the purpose of improving the tribological properties thereof, and also relates to surface-treated titanium and titanium alloys having improved tribological properties and uses for such surface-treated titanium and titanium alloys.
Over the past forty years, there have been many investigations into the effect of surface treatment of titanium and titanium alloys on surface hardness. A great deal of work has been devoted to the study of oxidation of titanium and its alloys which is generally viewed as a problem when surface treating titanium and its alloys in various gaseous environments. Little attention has been paid to tile deliberate oxidation of titanium alloys for use as a tribological surface treatment. Investigations have been reported in various journals over a long period of time. H. W. Worner in "Surface Hardening of Titanium", The Australasian Engineer, November 1950, pages 52 to 55, observed that, when commercially pure titanium was heated in the range of 850 to 1000.degree. C. in air at a pressure of between 10.sup.-3 mm and 10.sup.-2 mm Hg, the surface was effectively hardened. However, R. W. Hanzel in "Surface Hardening Processes for Titanium and its Alloys", Metal Progress, March 1954 pages 89 to 96 discounted the commercial utility of such process since, at a temperature high enough to achieve an appreciable hardening effect, a considerable amount of scale is formed and the fatigue strength is also reduced. At the other end of the temperature scale, it was subsequently shown that the coefficient of friction of commercially pure titanium markedly decreases when it is heated in air at 350.degree. C. for 17 hours; it was also shown that the coefficient of friction of the oxidised surface of titanium remains low after removal of brittle compound layers (see E. S. Machlin et al, Journal of Applied Physics, Vol 25, 1954 pages 576 to 581 and W. R. Yankee, "Influence of Oxygen and Nitrogen in Solution in Alpha Titanium on the Friction Coefficient of Copper on Titanium", Transactions AIME, September 1954 pages 989 to 990). However, such a procedure is costly since it requires the additional step of removal of such brittle layers.
In view of the difficulties associated with the severe scaling of titanium alloys when heated in air, the possibility of controlled oxidising in molten salts has been investigated. When titanium specimens are heated in lithium carbonate salt baths at temperatures between 600 and 900.degree. C. for 2 to 4 hours, satisfactory layers are said to be formed. The technique has been used for the production of batches of titanium pistons, as disclosed by E. Mitchell et al in, "Surface Treatments for Improving the Wear Resistance and Friction Properties of Titanium and its Alloys", journal of the Institute of Metals, Vol 93 1964/65, pages 381 to 386. Also, JP-A-56-146875 (Patent Abstracts of Japan, Vol 6, No. 24 (C-91) Feb. 12, 1982) discloses the formation of stable titanium oxide on a titanium material by burying the material in magnesium oxide or aluminium oxide and heating at 550 to 850.degree. C. in air.
The so-called Tifran process (see A. Goucher et al, "Nouvelles Possibilites de Frottement des Alliages de Titane: Le Tifran," Entropie, No. 63, 1975, pages 36-41) has been used to treat Ti-6Al-4V and involves gaseous oxidation of the titanium alloy at 750.degree. C. for 10 hours to produce a case depth of about 50 .mu.m. The process is reported to result in a surface layer having a titanium oxide base, and a diffusion zone. However, such process parameters produce a porous poorly adherent oxide layer and carry with them the risk that components of complex geometry would be distorted. In another form of the Tifran process, the titanium alloy is oxidised at 630.degree. C. for 3 hours. However, this produces a titanium dioxide layer of negligible thickness.
R. M. Streicher et al, "New Surface Modification for Ti-6Al-7Nb alloy: Oxygen Diffusion Hardening (ODH)", Biomaterials, Vol 12, 1991 pages 125-129 disclose graded oxygen diffusion hardening to a depth of 50 .mu.m with a maximum hardness of 900 HV compared with 360 HV for the untreated alloy. The ODH-treated alloy is claimed to have improved friction and wear resistance and to be useful in surgical prostheses. The corrosion resistance of the ODH-treated titanium alloy is claimed to be equal to that of commercially pure titanium and the untreated alloy. However, no parameters are described and the micrographs show no evidence of a TiO.sub.2 layer of a dimension with which the present invention is concerned.
M. Mushiake et al, "Development of Titanium Alloy Valve Spring Retainers", SAE Technical Report Series No. 910428, 1991 pages 41 to 49, disclose a wear-resistant surface treatment based on air oxidation to protect valve spring retainers made of Ti-22V4Al .beta. titanium alloys. A better wear resistance is said to be afforded to the component by using the oxidation process treatment as compared with either ion nitriding or gas carburising. M. Mushiake et al disclose that oxidation at 850.degree. C. for 30 minutes of such titanium alloy valve spring retainers imparts a better wear resistance than that of steel retainers. However, this process is not applicable to .alpha. or .alpha.+.beta. alloys since it alters the bulk microstructure, degrades the properties and risks causing problems of distortion, particularly for components of complex geometry.
WO95/09932 discloses the oxidation of a titanium alloy product to improve tribological properties by a procedure which involves deep surface hardening to a depth of greater than 100 .mu.m by localised surface re-melting without further alloying, optionally surface finishing the deep surface hardened material, oxidising to a depth of less than 100 .mu.m (usually less than 50 .mu.m and preferably in the range of 1-20 .mu.m), followed by modification of residual stress by shot peening or heat treatment. The above treatment improves rolling contact fatigue resistance and scuffing resistance. Thermal oxidation of the alloy product in air at 600 to 850.degree. C. produces layers of oxide and oxide-rich Ti at the surface. In one particular example, thermal oxidation in an air-circulation furnace for 10 hours at 650.degree. C. is performed as part of the previously described processing sequence which results in a very substantial improvement in wear resistance as compared with the completely untreated material.
A. K. Mishra et al ("Diffusion Hardening--A New Surface Hardening Process for Titanium Alloys", Surface Modification Technologies VII, The Institute of Materials, 1994 pages 453-471) refer in general terms to a procedure for diffusion hardening a Ti-13Nb-13Zr alloy which involves using a proprietary treatment in an atmosphere containing atomic oxygen, but without giving any process details. Treated specimens are said to have a 0.7 .mu.m surface layer comprised of ceramic oxides such as ZrO.sub.2, TiO.sub.2 and Nb.sub.2 O.sub.5 with an oxygen penetration depth of 2-3 .mu.m, and an increased surface hardness and abrasion resistance
It is an object of the present invention to provide an improved oxidation treatment which is simple to operate and which can produce titanium or titanium alloys having improved tribological properties such that the treated material is suitable for use in a relatively wide variety of applications.